HIGH-PRECISION CONDUCTOMETRIC DETECTOR FOR THE MEASUREMENT OF ATMOSPHERIC CARBON-DIOXIDE

A new, lightweight instrument has been developed for measuring atmosph eric mixing ratios of the greenhouse gas carbon dioxide with high prec ision and accuracy. This modified total organic carbon analyzer uses a bundle of semipermeable hollow fiber membranes to continuously equili brate CO2 in air with a recirculated stream of deionized water. Aqueou s carbon dioxide hydrolyzes and dissociates to form the ions H3O+ and HCO3-, thereby increasing the conductivity. The bipolar pulsed method is used to measure the conductivity of the water before and after cont act with air. Potential interferences that may also increase the condu ctivity, such as acids and bases, are removed at the inlet by bicarbon ate and bisulfate scrubbers. The detector has been field tested and ex hibits a 1/e response time of similar to 30 s and precision (RSD) of 0 .1%, Field results for measurements at altitudes in the range of 1700- 3600 m MSL exhibited an average deviation of 0.16% from values obtaine d by flask sampling and laboratory measurement using the well-establis hed method of nondispersive infrared absorption. High accuracy and pre cision combined with the ability to obtain real-time, continuous data make this detector well suited for evaluating landscape-scale carbon d ioxide fluxes from vertical profiles through the convective boundary l ayer.